S15 steering wheels are hard to find. Unlike S13's, the S15 is not a car from which many parts come, because of three reasons:
1) Because of the lack of applicability of the Japanese 10-year old car laws (the same laws that make S13 parts so readily available to us in America), no pressure is placed on Japanese S15 owners to buy a newer car. This means that very, very rarely is an S15 parted out.
2) When an S15 is parted out, parts like the steering wheel, seats, engine, transmission, projectors, etc. are quickly snatched up by buyers in Japan, so few of these parts make it to America.
3) The S15 is still a relatively new car, not owned often by young punks like myself. Demographically speaking, the S15 is often owned by a slightly older bracket of Japanese people, who are statistically a lot less likely to total their cars. This means that parts like the steering wheel, etc. are not often listed on the market. And when they are sold, option 2 often comes into play.
If you really really want to find an S15 steering wheel, I would just check EBay. I saw one on there this week for just $325.
Personally, I don't think it's worth it one bit. Just get one of the sportier Nismo knock-off steering wheels with hub for like $100.